Communication system



--April 1, v1941. A M. CURTIS E-r M. .2236524 colmunrcuzou SYSTEI Filled July s. 1959 SOD A Patented Apr. 1, 194.1

'UNITED 'STATES` PATENT OFFICE) azsaszr l COMUNICATION SYSTEMI Austen M. Curtis, South Orange, N. J., and Alfred 4ILMelliosc, St. Albans, N. Y., assignors'to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York 4Claims.

This invention relates to communication systems and particularly vto electric signaling systems involving a mutable link or a signaling channel capable of or liable to change from internal or external cause which may give rise to interfering energy or, more specically, subject to noise, fading or change of impedance.

The object of the invention is to insure the correct reception of signals .even under exceptional circumstances andl severe conditions of disturbance in the mutable link.

It is well known that radio circuits using both side-bands for the same transmission develop interaction between the side-bands when the carrier frequency fades. This interaction causes the fundamental modulating frequency to be converted into a tone of double that frequency and this phenomenon inthe past has been a source of annoyance and in some cases where elaborate filtering means have had t be provided, a costly annoyance. v

A feature of the present invention is the use of a narrow band-pass filterfor admitting to a signal responsive circuit, signals of a given narrow frequency band and in parallel therewith another band-pass filter for admitting to the said signal responsive circuit signals of double the given frequency band. Through such a novel arrangement it has been found that signals which might otherwise be lost lare saved, for when deep fading takes place so that the radio carrier frequency drops to such a low level that the fundamental modulating frequency bandv appears at the receiving apparatus at a level insumcient to perform its duty as a signal, the interaction of the two radio side-bands develops a telegraph signal frequency band of double the fundamental and at a level sufficiently high to act as a signal. Hence, a feature of the invention vis to make use of the .phenomenon which appears under these conditions in the mutable link and which was heretofore considered an annoyance, to fortify,

' or act in place of the signal which would otherwise disappear.

The invention is an improvement over the system shown in the Patent 2,207,720, `granted July 16, '1940 to Cole et al.

The drawing consists in a single sheet comprising a -block diagram giving a. general i schematic of the system and indicating the novel use of certain well-known instrumentalities.'

A 'speech path i leads'to a hybrid coil 2 from which a path 3 for outgoing speech leads to a vogad `I. The outgoing speech theny passes through a transmitting suppressor I and by This voice operated switch-v from the transmitting suppressor 5v through the delay and amplier 8 and the transmitting singing suppressor 9 to the hybrid coil i0. From the hybrid coil i0 the speech reaches another hybrid coil li and thence goes through the radio transmitter II2 to the transmitting antenna Through radio transmission speech will berreceived at some distant point and for purposes 'of explanation, it will be assumed that the distant pointV has a receiving circuit similar in all respects to the lower part of Fig. 1. The speech, therefore, enters antenna il, passes through a radio receiver I5 and a repeater I6 to a hybrid.

coil i1. From the hybrid coil I1 the speech goes into hybrid coil I8 thence through the receiving -singing suppressor i9, a receiving repeater 20, a

showing is by Way of example and that the te'rm' mutable link does not necessarily mean that a Vspace radio link-is invariably involved but that the term is broader in its meaning and includes any channel liable to change from internal or external cause.

'Ihe voice operated switching circuit 'l controls a control tone enabler 23 so that asource 'of control tone '24 is passed through the control tone lter 25 to the hybrid vcoil Il and thus accompanies speech. The voice operated switching circuit 'l also controls a switching point leading from the receiving switching circuit 26 at the same terminal to the transmitting suppressor S so as' to open this circuit during the periods when speech is going out from line l to -antenna i3. During this time the voice operated switch-A ing circuit 'l also controlsthe telegraph control circuit 21 to stop the action of the telegraph transmitting circuit.

' VAt the receiving end the control tone modulating the carier frequency in the usual manner coming in over antenna Il is demodulated in radio receiver I5 and passes through hybrid coil I1 and thence through telegraph band elimina.-

` tionfilter 2l and into a control tone amplinerand filter land thence into detectorlii This oberstes the relays of thereceivmg switching circuit 26 which performs several functions/as follows: First, the circuit 2l controls a normally open circuit between the speech amplifier Il and Y held operated thereafter by speech currents iiowing through the amplifier It. lThis means that the circuits 29 and 3l are both effective and operative in conjunction with each other to hold the receiving switching circuit chain of relaysl operated.

The receiving switching circuit 26 also controis the receiving singing suppressor Il to unblock the speech path upon reception of control tone. The receiving switching circuit 26 also controls a normally closed path between the band-pass filters 3| and 46 and the telegrad 22 so that while speech is being received the telegrad 32 will be disabled. At the same time the receiving switching circuit 26 controls the switching relays 33 for two telegraph channels.

During pauses in speech, either when the subscriber at the distant end -is talking into channel I or during the time when the subscriber on channel I pauses during his talk out over the antenna lIl, the telegraph apparatus shown in ythe upper part of the tlgure is in operation, This comprises perforators 34 and 35, working, respectively, into tape transmitters 36 and 31 and thence into rotary distributor I8. Rotary dis-- tributor 2B works into the control -circuit 21 which in turn exercises control over the telegraph signal suppressor 39. A source of telegraph tone is supplied to the telegraph signal oscillator 40 and through the action of the telegraph signal suppressor 39 under control of the control circuit sends telegraph signals into the hybrid coil III thence through the hybrid coil II and out over the radio transmitter I2.

At the distant end these telegraph signals in the form of spurts of alternating current of the usual form of a carrier oscillator frequency with side-bands about 75 cycles per second wide caused by the modulation of the carrier by the telegraph transmitter are received over antenna Il and `radio receiver I5 and thence pass through the hybrid coils I1 and I8 to the bandpass filters 3|` and I6. Thereafter, the signals enter the telegrad 32 and are converted from varying strength 4alternating current signals to uniform strength direct current signals of practically the same length at which 'they were received over the antenna I4.

The band-pass filter 3l, herein, for example, marked 2295 cycles is a narrow band-pass Iiilter the transmission of which is centered about the same frequency as the telegraph signal oscillator 4l. When double side-band transmission is used `of telegraph signals the receiving switching circuit2lisimoperatedandthespeechpathis blocked off by the receiving singing suppressor I0.

A line Il schematically represents the means for keeping the rotary distributor 2l in exact synchronism with the distant'rotary distributor 4I. The rotary distributor 2l and the rotary distributor 4I at the same terminal may or may not be kept in synchronism. In actual practice they generally are kept in synchronism for reasons having to do with the cost of synchronizing apparatus but from an operating standpoint it is only necessary that the rotary distributors formingpart of the same channel be kept synchronized.

The receiving singing suppressor may be of a simple nature, such as the device shown in Patent 1,749,851, granted March 11, 1930 to H. C. Silent or it may be of an improved typefully explained in application Serial No. 283,367, filed by Berger and Curtis on even date herewith.

What is claimed is: i

1. In a communication system, a signaling channel including a mutable link, means at the transmitting end of said channel for sending alternating current signals of a predetermined frequency band, means at the receiving end of said channel for receiving and translating alternating current signals. a narrow band-pass illter for admitting to said receiving circuit lsignals of said predetermined frequency band and in parallel therewith a narrow band-pass filter for` admitting to said receiving circuit signals of the second harmonic of said predetermined frequency band.

2. In a communication system, a signaling channel including a mutable link, means at the transmitting end of said channel for sending a carrier frequency modulated by signals of a predetermined frequency band, means' at the receiving end of said channel for detecting said signals, a circuit remonsive to alternating current signals, a band-pass lter for admitting detected signals of said predetermined frequency band to said signal responsive circuit and in parallel therewith another band-pass illter for admitting detected signals of the second harmonic of said predetermined. frequency band to said signal responsive circuit.l

3. In a communication system, a signaling channel including a mutable link, means at'the transmitting end of said channel for transmitting over the mutable link the interaction of the sidepass filter I6-hes been connected in multiple with the .band-pass filter 3| so that when the signals of the 2295 cycles band become so weak through fading that they would' otherwise fail to operate the telegrad 32, a substitute signal bland centered about 4590 cycles will operate telegrad .32 in its stead. l

The operation of the telegrad is fully explained in the application of Barney-Berger-Melhose Serial No. 283,368, iiled on even date herewith.

The direct current signals produced by the telegrad 22 pass over path 15 to the switching relays I3 thence through the rotary distributor Ii to the two printers 42 and 43. During the reception a predetermined frequency band of alternating current signals by modulated carrier double sideband means, a circuit at the receiving end of said channel responsive to alternating current signals, a narrow band-pass filter for admitting to said circuit signals of said predetermined `frequency band and -in parallel therewith another narrow band-pass filter for admitting to said circuit signals of the second harmonicof said predetermined frequency band generated in said mutable link.

4. In a communication system, a signaling channel including a mutable link, means at the transmitting end of said channel for sending alternating current signals oi' a predetermined frequency band, means at the receiving end of said channel for receiving and translating alternating current signals, and filtering means for admitting to said receiving circuit signals of said predetermined frequency band and the second harmonicv of said predetermined frequency band.

- AUSTEN M. CURTIS.

ALFRED E. MELROSE. 

